1. Field of the Invention
This invention relates to improved gypsum formulations, and more particularly to gypsum board formulations whereby increased protection against fire is attained.
Gypsum board products, comprising a monolithic core of set gypsum and a cover sheet (generally paper) encasement, are well known in the art. They are widely used in the construction of interior walls and ceilings and are variously termed gypsum panels, plaster board, gypsum wallboard or the like.
The chemically combined water (about 21% by weight of the gypsum) contributes to the effectiveness of products containing it as a fire barrier in various building and construction products. When gypsum board or set plaster formulations are exposed to fire, the water is slowly released as steam, retarding heat transmission for a time as the gypsum calcines. The heat resistive properties of various gypsum building materials have been determined by testing facilities on fire testing of assemblies performed in accordance with the American Society for Testing and Materials (ASTM) procedures. For example, ASTM C 36 Section 3.3 provides a special fire retardant designation, type X, for gypsum wallboard that provides at least one hour fire retardant rating for boards 5/8 inch (16 millimeters) thick, or 3/4 hour fire retardant rating for boards 1/2 inch (13 mm) thick, when the boards are applied to a test partition in single-layer nailed application on each face of load bearing wood framing members and the assembly tested in accordance with the requirements of ASTM method E 119.
From studies of the actions of gypsum board when exposed to a fire, such as in a laboratory fire test, it has been generally evident that there is a substantial shrinkage of the board core at sustained high temperature with consequent cracking, which not only contributes to passing excessive heat and hot gases through the test wall but also hastens the disintegration of the board under these adverse conditions. Also, as,the gypsum calcines it loses its inherent set strength.
2. Description of the Prior Art
To increase the fire resistant properties of these products it has been conventional to introduce certain fibers and unexpanded vermiculite ores into the slurry of calcium sulfate hemihydrate (plaster or calcined gypsum or gypsum stucco) and water during the board forming process. This concept is disclosed in U.S. Pat. Nos. 2,526,066; 2,681,861; 2,744,022; 2,803,575; 2,853,394; 3,454,456 and 3,616,173. These patents basically teach the use of certain unexpanded vermiculite to offset the shrinkage of the board core during the heat exposure, the unexpanded vermiculite expanding as the chemically combined water present in the gypsum is driven off. As this heating also tends to degrade the cohesiveness of the gypsum, reducing the strength and integrity of the core, the fiber component of the core formulation imparts a mechanical binding or matting effect to help hold the calcining gypsum together and keep it from disintegrating and falling into the test furnace.
Further, U.S. Pat. No. 3,616,173 notes that particular proportions of certain small inorganic particles will further improve the overall fire resistant properties of the board cores containing unexpanded vermiculite. Thus certain clays of less than 1 to about 40 micrometer (um) size and either colloidal silica or alumina of less than 1 micrometer size, or mixtures thereof, are said to provide some fire resistant properties in further cooperation with the ore fiber mixture. U.S. Pat. No. 3,454,456 indicates that having some proportion of the unexpanded vermiculite present as fine sized particles smaller than 100 mesh (147 um) helps to prevent large surface fissures and spalling on the board core. This patent calls for the use of an unexpanded vermiculite of a particle size which will pass through 50 U.S. Standard mesh (297 um) and be retained upon a 140 mesh sieve (105 um) for accomplishing low fire shrinkage and low spalling.